1. Field of the Invention
This invention relates to a multilayer coated corrosion resistant metal pipe, or a metal pipe having an outer surface coated with a plurality of layers of corrosion resistant materials. It is particularly concerned with a metal pipe having a relatively small inside diameter not exceeding 20 mm.
2. Description of the Prior Art
Metal pipes often have their outer surfaces covered with protective coating. These pipes are used for making, for example, pipelines for conveying a brake fluid and fuel in a motor vehicle. These pipelines are located under the body of the vehicle. As they are used in such a bad environment, the pipes are required to have high degrees of corrosion resistance, scratch resistance, impact strength and mechanical wear resistance. Spiked tires have come to be often used in the cold season during which ice is likely to form on the road surface. These tires, however, damage the road surface and raise dust when it is not covered with ice. Rock salt is sprinkled onto the road surface to prevent its freezing. A yearly increase in the use of rock salt, however, is making the problem of pipe corrosion more serious. The pipes are also liable to damage or wear by stones or mud spattered by the rotating wheels of the vehicle. It is, therefore, necessary that the pipes be so coated as to resist both chemical corrosion and mechanical damage or wear.
Reference is made to FIG. 3 showing by way of example a known pipe of the type to which this invention pertains. It comprises a double-rolled steel pipe 11 made by rolling a steel strip or hoop twice and brazing its longitudinal edges by means of a copper plating layer 12, or a seam welded steel pipe. The pipe 11 has an outer surface coated with an electroplated zinc film 13. The film 13 has an outer surface coated with a relatively thick special chromate film 14 having an olive color. The chromate film 14 has an outer surface coated with a fluorinated resin film 16. Pipes of this kind are disclosed in, for example, Japanese Patent Publications Nos. 60434/1982 and 23271/1986.
The fluorinated resin film is formed by impregnating the chromate film with a dispersion of polyvinyl fluoride immediately after the formation of the chromate film when it is still in the state of a gel, and drying them under heat, so that the fluorinated resin film may form an intimate bond with the chromate film. When the chromate film is formed by treating the pipe with a solution, it requires large amounts of a chromium compound and an organic acid, such as formic acid, used as a reducing agent. It is necessary to supply the treating solution with the chromium compound, etc. frequently and yet renew it at regular intervals of time in order to maintain a constant film forming capacity. The waste solution, however, contains a large amount of chromium having a valence of 6, which is a toxic substance, and its disposal, therefore, costs a great deal. Although the chromate film as formed is highly resistant to corrosion, the heat to which it is exposed during the formation of the resin film deprives it of water and thereby makes it brittle. The plastic deformation, such as bending or double flaring, of the pipe forms in the chromate film fine cracks which lower its rustproofing property.